if you click the start button and type %appdata% a folder called "roaming" should pop up. Inside the roaming folder you will find the .minecraft folder. That is the main directory for minecraft, so deleting that and reinstalling the new client(1.6.2) should allow you to re-enter your e-mail/login. That, of course, will not fix any issues you are having with registering your minecraft account. Double check your e-mail and see if you got a confirmation e-mail, you might not have confirmed or something simple. If that doesnt work, you will need to contact minecraft support inorder for them to active your account.